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  Current Research Highlight :: October 2012 all highlights

Smashing white dwarfs: explaining the brightness of cosmic explosions

Supernovae are among the brightest and most energetic events to occur in nature. However, the origin of a particular type of supernova - the Type Ia supernova - is still a mystery despite decades of research. What type of stars produce these explosions, and how? Researchers at the Max Planck Institute for Astrophysics in Garching, the Australian National University in Canberra, Heidelberg ITS and collaborators have investigated a particular explosion model involving merging white dwarf stars. They found that the explosion brightnesses from the merger models are strikingly similar to the range of explosion brightnesses that is observed for real Type Ia supernovae. This means that violent white dwarf mergers might be a dominant formation channel for these explosions.

Fig. 1: An artist's impression of merging white dwarfs. Such mergers are thought to be potential progenitors of Type Ia supernova explosions.
Image courtesy of (c) Nature 2010

Fig. 2: Model peak brightness distribution of merging white dwarfs for a range of allowed mass ratios (coloured lines). The observed peak brightness distribution from Type Ia supernovae (from Li et al. 2011) is shown in grey-scale. The observational data have been scaled up in order to easily compare the distribution shapes. The theoretical brightness distributions cover the range and match the shape of the observed distribution fairly well.

Fig. 3: This plot shows the number of violent white dwarf mergers as a function of time (from 100 million to 10 billion years after the stars are first born). The blue line represents the violent white dwarf merger model (cf. the blue histogram in fig. 2). Red squares and the black line show the recovered 'delay time distribution' of SNe Ia from two observational studies (see references at the end of the article). The violent white dwarf merger rate matches the fit from GM12 extremely well, implying that there might be enough violent mergers to account for a large fraction of SNe Ia in field galaxies.

Type Ia supernovae (SNe Ia), which make up about one quarter of all supernovae, are believed to come from exploding white dwarf stars, though how the white dwarf reaches the critical conditions to make it explode is still unclear.

More than 95% of stars will end their lives as white dwarfs (including our Sun when it runs out of fuel), but only a small fraction of these will actually explode. A lonely white dwarf star is stable - it won't spontaneously erupt. However, if there is a source of matter nearby - e.g. another star - the white dwarf can steal mass from this companion, with explosive consequences. Thus, astronomers have been trying to find out what types of double star systems including at least one white dwarf can lead to the formation of Type Ia supernovae.

Since white dwarfs are rather faint when they are not exploding, observations alone cannot solve this well-known 'progenitor problem'. Therefore testing of theoretical models has become a critical step in understanding the origin of SNe Ia.

The biggest mystery shrouding SNe Ia is this: what type of star is 'donating' mass to the white dwarf? Is the companion a normal (Sun-like) star tranquilly passing matter to the white dwarf, thereby slowly pushing it closer and closer to the critical limit, or is it another white dwarf star that violently smashes into the more massive one, immediately causing an explosion?

Using a detailed model for the evolution of double stars, state-of-the-art hydrodynamic explosion models and a sophisticated method for predicting how the energy from the explosion is turned into observable light (spectra), MPA researchers and collaborators determined that white dwarfs which smash violently into each other give rise to a range of brightnesses that matches the range in brightness that is actually observed for Type Ia supernovae. Even more encouraging, the model brightness distribution peaks at about the same value as the one from observations (see figure 2). Any model scenario that is claimed to account for a large fraction of SNe Ia must be able to explain observational trends. Not only does the violent merger model do very well in terms of reproducing the brightness distribution of real SNe Ia, it also produces the right number of events as a function of time (the 'delay time distribution', see figure 3).

In this particular model the peak brightness of the explosion is directly related to the mass of the more massive (primary) white dwarf. To get a typical explosion, however, most primary white dwarfs have to grow in mass before they explode. The team has identified an evolutionary pathway which serves to 'beef up' the mass of the primary well before the merger occurs. However, it has yet to be confirmed whether white dwarfs can really be 'beefed up' by their companions sufficiently in large enough numbers.

While the MPA researchers are excited about their result, they remain slightly cautious. It is still unclear if this formation scenario of pre-merging white dwarfs is realized in nature as efficiently as the binary evolution model indicates. Some further work and (probably) future observations are needed to confirm the various aspects of the model.

If it turns out that such an 'evolutionary channel' that leads to more massive primary white dwarfs readily contributes to making white dwarf pairs, then it is likely that violent white dwarf mergers are driving the underlying brightness distribution of SNe Ia. If not, then some other explosion scenario could be dominating the SN Ia scene.

Ashley Ruiter, Stuart Sim, Ruediger Pakmor, Markus Kromer, Ivo Seitenzahl, Stefan Taubenberger

Original publication

Ruiter, A. J.; Sim, S. A.; Pakmor, R.; Kromer, M.; Seitenzahl, I. R.; Belczynski, K.; Fink, M.; Herzog, M.; Hillebrandt, W.; Roepke, F. K.; Taubenberger, S. "On the brightness distribution of Type Ia supernovae from violent white dwarf mergers", submitted to MNRAS.The draft is available on astro-ph: linkPfeilExtern.gif

Further references

linkPfeilExtern.gifLi et al. 2012
linkPfeilExtern.gifMaoz et al. 2012 (MMB12)
linkPfeilExtern.gifGraur and Maoz 2012 (GM12)

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